Electrodes have been produced by deposition of conductive ink materials onto solid substrates. Such devices may comprise a solid layer of a precious metal, e.g. gold or platinum, on a ceramic substrate. They may be produced by, for example, thick film deposition (TFD) of an ink material loaded with finely-divided particles of the metal; drying the ink layer at extreme temperatures (usually in excess of 500.degree. C.) removes all ink components such as binders and solvents, and fuses together the particles of precious metal, to form a continuous film of metal on the substrate.
The performance of such devices as electrode materials very closely approximates to that of ideal materials, i.e. solid, machined pure metal electrodes of (say) gold and platinum characterised by high conductivity, high sensitivity to the analyte of interest, low noise levels, and a response to the analyte which is relatively independent of temperature. However, such devices are expensive to produce, since the ink materials must have extremely high metal loadings, and require expensive high temperature drying/curing facilities.
Alternative electrode materials have been described, which are fabricated by TFD of an ink material consisting of finely-divided noble metal particles, intimately mixed with or deposited on carbon, a resin binder material and, if required, a solvent. Such devices are considerably less expensive than those described above, since the ink materials contain much lower levels of noble metal and are fabricated by low temperature curing of the ink material; the carbon content of the ink is designed to compensate for the loss in conductivity that results from reduction of the noble metal content.
Such carbon-based devices are described in, for example, GB-A-2191003. They suffer from three major problems. Firstly, the noble metavcarbon mix is highly heterogeneous, so that products suffer from very poor precision due to the non-uniform dispersion of the noble metal in the ink material. Secondly, there are high background currents during analysis (cf noise), resulting from high surface areas of the carbon components; non-faradaic components, i.e. double layer charging currents. are a direct function of the carbon particle surface area. Thirdly, the temperature dependency of the background current increases with increasing temperature; this is usually a function of the flexible nature of the polymer binder, which essentially softens with increasing temperature, thus changing the surface topography of the electrode.
Diabetes is the most common endocrine disease, and has major deleterious health consequences for the sufferers. Disease morbidity may be decreased by a program of blood glucose monitoring in which patients use samples of blood to monitor their blood glucose levels and adjust diets, drugs and insulin therapy according to the level of blood glucose. In extreme cases, such monitoring may be used to avoid hypoglycemic attacks which may cause coma and subsequent death. However, blood for analysis is obtained either by a fingerstick or venous sample, which causes patient pain and discomfort. Sweat-collecting devices are known. These are usually skin patches made of a hydrogel containing permeation enhancers. The collected sweat or exudate is either placed in water to allow the glucose to diffuse out of the hydrogel and then be analysed or the skin patch is allowed to concentrate the sweat in the patch by driving off the collected water and a specific binding partner in the patch is used to present a visual indication of its presence in the patch. These methods are both qualitative. In addition, the patient has to carry out several manipulations which would be particularly difficult if the patient is ill or entering hypoglycemic coma.
GB-A-2191003 (see above) discloses that an enzyme such as glucose oxidase should be adsorbed or immobilised on the resin-bonded carbon or graphite particles of an electrode. For example, enzyme is held within the pores of the oxygen-permeable resin-bonded layer.
WO-A-97028 11 discloses a hydrogel patch containing glucose oxidase. This can be used with a standard electrode, as a glucose sensor.